In a filling process of producing packaged beverages (canned beverages, beverages in PET bottles, bottled beverages), a beverage is typically flowed from above into a container disposed vertically in a filling machine, then the container is joined to a lid member and sealed in a sealing machine (a seaming machine, a capper and the like). In order to maintain the beverage quality and to improve flavor, it is important to reduce the amount of residual oxygen in the sealed container. The removal of oxygen from the head space inside the container is especially important. A deoxidation technique, such as undercover gas displacement, performed immediately prior to sealing has been developed, is used to realize such removal. On the other hand, because packaged beverages are products consumed in large volume, the rate of the filling process has been increased, and, in the case of canned beverages, high-speed lines capable of producing 1000 to 2000 cans per minute have seen practical use. Following the beverage filling process, foam is generated inside a container. Foam generation behavior and disappearing behavior of the generated foam differ depending on the properties of an individual beverage and the filling conditions, but typically when the production rate is high, a large amount of foam is generated, available time is insufficient to destroy foam, and foam remains during sealing.
Foam contains oxygen at the same concentration level as air, and because the oxygen contained in the foam cannot be removed by gas displacement in the head space, the foam makes it difficult to decrease the amount of oxygen in the head space. In particular, at present since the level of deoxidation technology used for gas displacement has advanced, the remaining oxygen is primarily due to foam. Currently mixing a defoaming agent to a beverage preparation is generally employed to inhibit foam, but because a defoaming agent affects beverage taste, a technology capable of defoaming after filling and before sealing is required.
A method for defoaming by light irradiation has been considered as a means for resolving this problem, and a large number of methods and devices related to this technology have been suggested (see, for example, Patent Documents 1 to 4). Also methods for laser beam irradiation have been suggested. For example, a method for defoaming with laser radiation has been suggested, where intermolecular bonds forming a foam film and organic molecules or water molecules contained in the film are caused to oscillate and are excited by irradiating foam with a laser beam, and the intermolecular bonds are broken down and foam is destroyed (see Patent Document 4).
Methods and devices for defoaming by irradiation with ultrasonic waves have also been suggested (see Patent Documents 5 to 9). Defoaming devices for generating shock waves by arc discharge have also been suggested (see Patent Document 10). Further, in addition to the methods and devices using light, sound and electric discharge, those using heating, microwaves, high frequencies, electric winds, electrostatic charges and vapors have also been suggested.
However with conventional defoaming methods using light irradiation, the defoaming rate is low, and does not match with increased filling speeds. As a result, such methods have not found practical use. These light irradiation techniques are based on the principle of irradiating foam with light energy, heating and evaporating foam components (mostly water), thereby destroying foam. Therefore an individual bubble must be directly irradiated with light energy, and because in general bubbles are distributed over the entire liquid surface, substantial light energy must be supplied to the entire liquid surface to destroy foam by light irradiation. The term “substantial” used herein means that either the light beam diameter is expanded to irradiate the entire surface at the same time, or that the irradiation is performed by scanning the liquid surface with a light beam of a small diameter. In both cases, the total light energy represented by (irradiation power per unit surface area)×(irradiation time)×(irradiation area) is the same. Because time allocated for the defoaming process in the accelerated filling process is short, a sufficient defoaming effect cannot be obtained with a light intensity obtained with a standard light source. Conversely, a high power light source is needed to produce energy sufficient to attain the defoaming effect, and such a light source is impractical.
On the other hand, when defoaming employs irradiation with ultrasonic waves, for example, if defoaming is performed by irradiating with continuous ultrasonic waves generated by an external sound source, because the wavelength of ultrasound is of the same order as the size of the container, that is the irradiation object, the waves lack directivity, irradiation energy is dissipated, and efficiency is poor. As a result, defoaming must be performed over a long distance on a conveyor, and the production line becomes long. Accordingly such an approach is undesirable from the standpoint of saving space. Furthermore in the case of narrow mouth containers, such as PET bottles or bottle cans, it is difficult to transmit light, sound, etc. to the entire liquid surface through the mouth opening, hence there is no effective defoaming means. Another problem associated with conventional methods is that it is virtually impossible to destroy foam that adheres to the inner surface of the container. In the case of arc discharge, the arc electrode breaks down over time. This makes electrode life short, therefore the arc electrode must be replaced periodically. This replacement operation contaminates the filling space, and if the filling is asceptic, sanitation is negated. The electrode material also causes scattering, and contamination is caused by the scattering as well. Furthermore, the discharge position is unstable, which results in dispersion of the defoaming effect. Particularly in such a moist environment as a filling line, discharge behavior may fluctuate considerably by the subtle difference in the humidity of gas between electrodes.
On the other hand, if a light having high energy is condensed in gas, an electrolytic dissociation phenomenon may occur in the gas at an irradiation point on the light propagation path, whereby pulsed sound waves can be generated. This breakdown induced by condensing laser beams is called “laser induced breakdown” (LIB).
The present inventors have discovered that a laser induced breakdown exhibits a superb defoaming effect, which provided the inventors with “a defoaming method characterized in that a pulsed laser beam is condensed and irradiated onto a gas portion above the liquid surface, so as to generate pulsed sound waves from an illumination point as a sound source, and foam is destroyed by the pulsed sound waves which propagate as spherical waves” (Patent Document 11).
According to this method, the pulsed sound waves cause a strong pressure change from the sound source propagating as spherical waves, destroying foam. Unlike the case of conventional defoaming by irradiating such light as a laser beam, it is unnecessary to irradiate each bubble with a beam, therefore defoaming is accomplished at high-speed in a short time. Since pulsed sound waves propagate all the way to the inner peripheral surface of a container, foam adhering to the inner peripheral surface of the container, of which defoaming has been difficult with conventional methods, can be effectively destroyed. If breakdown for defoaming is generated by condensing pulsed light in a gas portion above a liquid surface, shocks on the liquid surface are less than the case of directly irradiating the laser onto the liquid surface, and liquid drops scattering and adhering to a device, or adhering to a glass member of a laser irradiation hole, can be prevented. As a consequence, this method is advantageous in terms of device sanitation and maintaining optical characteristics.